The invention relates to delivery of a liquid composition to a substrate surface to form a coated layer thereupon, more particularly to a method and apparatus for preparing a coating hopper prior to initiation of delivery of a composition to a substrate surface, and most particularly to such a method and apparatus wherein composition is introduced into an empty coating hopper.
In forming a flowing sheet of a liquid composition for coating onto a substrate surface, the shape of flowing liquid composition is reconfigured from flow through a typically cylindrical conduit to flow though any of a variety of apparatus that create a sheet flow. These apparatus for creating a sheet flow are well known in the art and include, for example, a die, a distributor, an extruder, a weir, a slide surface, and a hopper. As used herein, all such types of apparatus are referred to collectively as hoppers. A hopper may comprise one or more parallel longitudinal members (typically referred to as hopper bars in the art) which are oriented transverse to the direction of liquid flow, which members may be bolted together or otherwise attached to form a hopper unit. A primary member may be referred to as a xe2x80x9chopper body,xe2x80x9d and one or more secondary members as xe2x80x9chopper bars.xe2x80x9d Typically, hopper bars are configured on their mating surfaces in such a way that internal flow passages for the composition are formed within the hopper when the bars are assembled together. Within a hopper, a flow path for liquid composition typically includes (in flow sequence) an inlet, one or more transverse distribution voids known as cavities, and a slotted exit from each cavity communicating with either a successive cavity or the exterior of the hopper. The last such slot is commonly known as an exit slot. Alternatively, a hopper distribution apparatus may include a distribution chamber open at the top and having a wall forming a weir for overflow cascade or curtain coating therefrom, the wall and weir being within the scope of the current invention.
In an extrusion hopper, the downstream end of the exit slot typically defines a coating lip from which the extruded sheet of composition is transferred directly to the passing substrate. In slide hoppers, as are used typically in the manufacture of photographic films and papers, composition is extruded from the exit slot onto an inclined slide surface terminating at a lower edge in a coating lip. The extruded sheet flows down the slide surface under gravity and is transferred to the passing substrate either through a dynamic bead, as in bead coating, or a falling curtain, as in curtain coating.
It is well known in the art that bubbles or particulate debris may be carried into or formed within a hopper and can become lodged in the composition flow path at any of numerous locations in or on the hopper. These bubbles and/or particulates can subsequently cause flow disturbances during coating resulting in unacceptable and continuous thickness variations in the coating as applied to the substrate. Further, such bubbles and debris may become dislodged during coating and be transferred to the substrate, resulting in unacceptable discontinuous thickness variations. Thus, it becomes very important that all debris and bubbles be eliminated from a hopper prior to commencing a coating operation. A strategy in the known art for accomplishing this is to conduct any of various rigorous hopper cleaning protocols using flush water. Typically, these protocols involve supplying particle-free and bubble-free flushing water through a coating composition/water manifold and valve arrangement positioned proximate to the coating hopper. The particle-free and bubble-free flushing water is pumped continuously through the hopper to a drain, the hopper being out of coating position. Mechanical devices such as plastic picks may be inserted into the hopper and agitated to assist in dislodging bubbles and composition residues from prior coatings into the flush water. Typically, such cleaning of a coating hopper used to coat radiation-sensitive coatings is conducted under white lights during a delay or product change in the coating operation. Flush cleaning may proceed for several minutes or more, until an operator is satisfied that no further composition or bubbles are exiting the hopper, and that the hopper is ready for introduction of composition.
One common method of coating hopper preparation used in the photographic coating art is to flush the hopper with water to displace air from the hopper much as described above. Once the flushing with water step is completed then coating composition is used to displace the water from the hopper, resulting in a hopper that is apparently ready for coating. Ideally, after all air and particulates have been displaced from the hopper, liquid coating composition is introduced into the hopper through the coating composition/water manifold and valve arrangement mentioned above. Specifically, the flow of flush water is stopped and the flow of liquid coating composition is begun. The liquid coating composition then begins to displace the water. Because the coating composition typically is an aqueous gelatin solution or emulsion, and because flow is not laminar through much of the flow path, the displacement typically does not occur as plug flow but rather there is substantial mixing of the coating composition with the residual water in the hopper. In good practice, the hopper may not be moved into coating position and coating may not commence until substantially all the residual water is eliminated from the hopper, and the slide surfaces and coating lip are observed to be conveying composition with absolute visual uniformity and no dilution.
For multiple-slot hoppers that simultaneously deliver a plurality of superimposed layers of coating compositions to form a multiple-layer composite coating, it is typical that the individual slots in the coating hopper are changed over from flush water to coating composition sequentially. It is also typical that the lower-most hopper slot is changed over first and the higher coating slots are changed over in sequence moving up the slide hopper surface. However, because in some facilities and coating applications, the top layer is more critical than the lower layers, it is sometimes preferable to purge the upper-most hopper slot first and purge sequentially down the hopper to purge the lower slots. The quality of hopper preparation for each slot is confirmed before the next slot is changed over. Thus the lower compositions may be flowed to drain for an extended period of time before the hopper preparation is complete.
In the practice of the method of the present invention purging of all of the hopper slots can also be performed simultaneously. To purge simultaneously the coating compositions flowing to each slot should reach the slide surface of the slide hopper at about the same time. Purge flow rates, coating composition delivery line lengths and head losses, and coating composition viscosity and density will all have to be considered if a simultaneous purge of all of the coating slots is contemplated.
Further, each composition delivery system typically is changed over from flush water to composition flow at a purge flow rate (which is significantly higher than the actual coating composition flow rate) to displace water more rapidly. The length of time required for the purge flow step and the actual flow rate during the purge flow step is typically empirically determined for each coating hopper and the various coating compositions used therein. However, in all cases, a large amount of coating composition may be wasted in displacing water from the hopper. Therefore, the known art hopper preparation method is costly, both in terms of lost machine time and in terms of coating composition waste.
What is needed is an improved method whereby a clean hopper can be filled with coating composition and prepared for coating initiation in a shorter time and with reduced composition waste as compared with the known art method.
It is therefore an object of the present invention to reduce the amount of coating composition wasted in preparing a coating hopper prior to initiation of coating.
It is a further object of the present invention to reduce the time required to prepare a coating hopper prior to initiation of coating.
Yet another object of the present invention is to prevent the starting of a coating with composition which is diluted with water due to incomplete purging of residual flush water.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished preferably by flowing flush water through the coating hopper (which may be a slide hopper or an extrusion hopper), the flush water filling the internal passages and exiting an exit slot or die, draining the flush water from the coating hopper, and flowing a coating composition at a purge flow rate through the coating hopper and across the external surfaces of the hopper normally wetted by coating composition. The purge flow rate may be greater than, equal to, or less than an actual or predetermined coating flow rate depending on a number of factors discussed hereinafter. With a slide hopper coating operation the flush water also covers the slide surface of the coating hopper. Alternatively, the method of the present invention may be practiced by flowing a coating composition through the coating hopper at a purge flow rate which is greater than an actual or predetermined coating flow rate while not performing a precursor water flush step at all.
The purge flow rate for purging with coating composition will vary from system to system. Although the purge flow rate will generally be greater than the actual or predetermined coating flow rate, the purge flow rate is actually dependent upon a number of factors. These factors include the viscosity and density of the coating composition, whether or not the coating composition is Newtonian in nature, whether or not the coating composition contains a surfactant, and the internal geometry of the coating hopper. A higher viscosity will generally allow for purging to take place at a lower purge flow rate. Similarly, a higher density will generally allow for purging to take place at a lower purge flow rate. As to whether or not the coating composition is Newtonian, Newtonian fluids are generally better for purging air.
Further, it should be appreciated that if the components of the coating delivery system are more difficult to purge of air than the coating hopper itself then such components should be positioned upstream of the coating hopper drain valve. Also, that portion of the coating supply line from hopper drain valve up to the hopper should be oriented to have at least some vertical slope component such that the drain valve is at the lowest position and such that there is no horizontal or sagging section in the supply line between the drain valve and the hopper. Horizontal lines and lines with sags are an obstacle to the air purging process.
The method of the present invention is useful in providing uniform coatings of liquid compositions to moving webs.